Many military systems rely on sensitive optical detectors for gathering information. Forward-looking infrared radars (FLIR), night-vision goggles, and the human eye (perhaps aided by a telescope or binoculars) are all examples. Image intensifiers of the old cascade tubes are giving way to microchannel plates providing a new range of ultrasensitive intensifiers. For example, in Jervis, M. H., and Needham, M. J., Image Intensifiers and Infrared Detectors, British Electro Optics, edited by Baker, L. R., published by Taylor & Francis, Ltd., London, 1977, at 31, several new highly sensitive devices are disclosed.
Along with this newly developing technology are newly developing problems. One such problem is that these systems suffer damage from bright sources that operate in the detector's band. One such bright source is a hostile laser. Protection against these countermeasure weapons have been rudimentary and either use a filter network to exclude the band of wavelengths containing the bright source or employ a shutter to close off input when it contains a high intensity component. A very fast shutter is needed. Such a shutter should be capable of sensing a threat and closing in nanoseconds because that is the potential rise time of an optical threat.
Problems remain with both a filter and shutter method of protection. One problem is that the filter excludes a band of the spectrum being received and the shutter closes the input signal completely off. Another problem is the new, agile laser weapon which covers an entire spectrum of frequencies. No protection exixts in the art today that can protect from changing frequency hostile lasers and still provide an optical input to the receiver.
The optimum protection circuit should discriminate and protect against dangerously intense sources without closing the input field, should react to any and all frequencies of high intensity energy and must have a response time that intercepts the high intensity source during its rise time before damage to the eye or sensitive equipment can occur.